Depletion region surface effects in electron beam induced current measurements

摘要

Electron beam induced current (EBIC) is a powerful characterization techniquewhich offers the high spatial resolution needed to study polycrystalline solarcells. Current models of EBIC assume that excitations in the $p$-$n$ junctiondepletion region result in perfect charge collection efficiency. However wefind that in CdTe and Si samples prepared by focused ion beam (FIB) milling,there is a reduced and nonuniform EBIC lineshape for excitations in thedepletion region. Motivated by this, we present a model of the EBIC responsefor excitations in the depletion region which includes the effects of surfacerecombination from both charge-neutral and charged surfaces. For neutralsurfaces we present a simple analytical formula which describes the numericaldata well, while the charged surface response depends qualitatively on thelocation of the surface Fermi level relative to the bulk Fermi level. We findthe experimental data on FIB-prepared Si solar cells is most consistent with acharged surface, and discuss the implications for EBIC experiments onpolycrystalline materials.